The invention relates to a method for coating surfaces, for which a precursor material is caused to react with the help of a plasma and the reaction product is deposited on the surface, the reaction as well as the deposition taking place at atmospheric pressure.
In the case of conventional plasma coating and plasma polymerization methods, the material is deposited on the workpiece, which is to be coated, under a vacuum or at least a pressure, which is greatly reduced in comparison to atmospheric pressure. These methods therefore require a major expenditure for equipment and are therefore not economically feasible for many practical applications, particularly since the workpieces, which are to be coated, usually cannot be brought continuously into the vacuum chamber and, instead, must be introduced batchwise. With regard to coating mass-produced products relatively inexpensively, a method would therefore be desirable, which has the known advantages of plasma coating or polymerization methods and therefore, in particular, enables very thin layers to be applied selectively with an exact composition and a defined profile of properties and, at the same time however, can be carried out under atmospheric pressure.
In a publication by R. Thyren: xe2x80x9cPlasma Polymerization at Atmospheric Pressurexe2x80x9d, Frauenhofer-Institut Schicht und Oberflxc3xa4chentechnik (IST), Braunschweig, a method is proposed for this purpose, for which the atmospheric plasma is produced with the help of a corona discharge. The corona discharge takes place between a working electrode, which has a dielectric as discharge barrier, and a counter electrode, which is disposed at the rear of the workpiece. The gaseous precursor material is supplied with the help of a so-called gas shower to the discharge gap between the working electrode and the workpiece. However, with this method, only moderate coating rates of the order of 10-20 nm/s can be attained. A further disadvantage consists therein that the plasma is formed only in the very narrow discharge zone between the working electrode and the workpiece or the counter electrode, so that the working electrode must be brought close to the workpiece, with the consequence that the distance between the working electrode and the workpiece represents a critical process parameter, and that the electrode configuration must frequently also be adapted especially to the respective geometry of the workpiece.
It is an object of the invention to provide a method of the type named above which, while easily carried out, makes an efficient and readily controllable coating possible, and to describe an appropriate device for carrying out this method.
For the inventive method, a plasma jet is produced by passing a working gas through an excitation zone and the precursor material is supplied to the plasma jet separately from the working gas.
Owing to the fact that, pursuant to the invention, the atmospheric plasma is generated in the form of a jet, which has a significantly greater range then the discharge zone of a corona discharge, the coating process can be carried out simply in that the plasma jet brushes over the surface of the substrate, which is to be coated. Since a counter electrode at the rear of the substrate is not required for this purpose, the workpiece may also be thicker and/or of complex shape. Since the precursor material is supplied separately from the working gas and fed into the plasma jet, which develops only in the excitation zone, the precursor material itself need not cross the whole of the excitation zone. This has the important advantage that the precursor material, which generally consists of monomeric compounds, is not decomposed or otherwise changed chemically in the excitation zone. For the desired reaction, which leads to the deposition of a polymer-like coating on the surface of the substrate, the number of reaction partners available is therefore significantly larger than in the case of the conventional method. Because of this effect, surprisingly high coating rates can be achieved, which can exceed the coating rates, which could previously be achieved with atmospheric plasma, by a factor of more than 10. The selection of the site, at which the precursor material is supplied, in relation to the excitation zone and the surface of the substrate, represents a process parameter, with which the coating process can become controlled sensitively. Sensitive precursor materials can be supplied in the relatively cool plasma jet downstream from the excitation zone. The low temperature of this plasma jet enables the precursor materials, which are stable only up to temperatures of 200xc2x0 C. or less, to be coated efficiently. The required excitation energy for the desired reaction of the monomers is provided primarily by free electrons, ions or free radicals, which are still contained in great numbers in the cool plasma jet. The further the site of supplying precursor material is displaced upstream in the direction of the excitation zone, the higher is the concentration of reaction-promoting ions, free radicals, etc. If the site for supplying the precursor material is shifted into the downstream region of the excitation zone, direct excitation of the monomers is also possible to a certain extent. In this manner, the excitation conditions can be optimized for the particular precursor material used. In general, an advantage of the inventive method consists therein that the processes of plasma generation on the one hand and of plasma excitation of the precursor material on the other take place in different zones, which overlap spatially only partially if at all, so that mutually harmful effects can be avoided.
The precursor material need not necessarily be supplied in the gaseous state and can, instead, also be supplied in the liquid or solid, powdery state, so that it evaporates or is sublimed only in the reaction zone. Likewise, it is possible to add to the precursor material solid particles, such as dye pigment or the like, which are then embedded in the polymer-like layer, which is deposited on the substrate surface. The color, roughness or electrical conductivity of the coating can be adjusted, as required, in this manner.
For feeding the precursor material into the plasma jet, it is also possible to use the Venturi effect in order to aspirate the precursor material into the plasma jet. On the other hand, if the precursor material is supplied actively, the extent of mixing of the precursor material with the plasma can be influenced selectively by the choice of the angle, at which the precursor material is supplied to the plasma jet.
Correspondingly, in the case of a spiraling plasma jet, the precursor material can be supplied in the same direction as the spiral or in the opposite direction.
If the desired reaction of the precursor material must take place in a reducing or inert atmosphere, it is possible to surround the plasma jet from the outside with a suitable protective gas, so that the reaction zone is separated from the surrounding air by a protective blanket of gas.
If a particular temperature is required for the desired reaction, this temperature can be achieved, for example, by heating the working gas and/or by heating the opening of the plasma nozzle.
For producing the plasma jet, a plasma nozzle can be used, which is similar, for example, to that described for other purposes in DE 195 32 412 C2. For coating larger surfaces, it is possible to dispose one or more such nozzles eccentrically on a rotary head (EP-A 986 939). Likewise, it is possible to use a rotating nozzle, which delivers the plasma jet at an angle to the axis of rotation (DE-U-299 11974).
For generating plasma with such a nozzle, it is possible to differentiate roughly between three areas: (a) the area of the arc discharge, in which direct plasma excitation takes place, so that there is strong excitation but also destruction of monomers, (b) the area of indirect plasma excitation, in which there is almost no destruction of the monomers, which nevertheless are excited efficiently and gently, and (c) a mixed area, which is characterized by little destruction and strong excitation of the monomers.